Feed is one of the most significant costs involved in animal production, and feed efficiency is therefore a very important genetic trait in animal agriculture. In poultry, a clear link between breast muscle mitochondrial function and feed efficiency has been reported. This chapter provides an overview of the mitochondrial processes which occur in muscle cells, presents the evidence that enhanced mitochondrial functions lead to high feed efficiency, and then considers the role of enhanced nucleotide metabolism and muscle cytoarchitecture in the feed efficiency of broilers.